When a heater is placed in a gas stream, the upstream side of the heater is cooled, while the downstream side is warmed by heat carried by the gas stream. It is thus possible to detect the gas velocity by measuring the temperature difference between the upstream and downstream sides of the heater. A flow sensor has been proposed, in which a temperature sensor is disposed on each of the upstream and downstream sides of a micro-heater (as disclosed in, for instance, Japanese Patent Laid-Open No. 193019/90).
In the conventional flow sensor, a thin film supporting a heater had large openings formed on the upstream and downstream sides to let both its upper and lower surfaces be exposed to the gas stream. These large openings cause turbulent flow of gas. Therefore, the relation between the temperature difference and gas velocity can not be obtained from a simple formula, but a cumbersome calibration is required.